1. Field of the Invention
The present invention relates generally to a spindle apparatus of a machine tool and, more particularly, to an improvement of a tool mountability in a tool mounting unit.
2. Related Background Art
In a machine tool in a machining center (MC) equipped with, e.g., an automatic tool changer (ATC), as illustrated in FIG. 6, a taper hole 2 for mounting a tool is formed in a front end of a spindle shaft 1. A taper shank portion 4 of a tool holder 3 is fitted into this taper hole 2, thereby mounting a predetermined tool. This type of taper shank portion 4 is classified into a BT type having a long taper prescribed by Japanese Industrial Standards (JIS) and a short taper type prescribed by German Industrial Standards (DIN). FIG. 6 shows an example of the BT type. Only at this type of taper shank portion 4, the tool holder 3 is supported on the spindle shaft 1, and it can not be said that the rigidity is so high. The entire surface of the taper hole 2 is not in well-contact with the entire surface of the taper shank portion 4, and, therefore, heavy cutting is hard to perform. Further, there appears such a phenomenon that a well-cut surface is hard to obtain in the case of finishing cutting.
In recent years, however, a new type of spindle apparatus as illustrated in FIG. 7 has come to be proposed in order to obviate such problems. With respect to the hold of the tool holder 3, this spindle apparatus takes a taper/end surface double-contact method of providing not only a contact between the taper shank portion 4 and the taper hole 2 but also a contact between an end surface 3a of the tool holder 3 and an end surface 1a of the spindle shaft 1.
In the case of the spindle shaft according to the above taper/end surface double-contact method, however, it is quite, as a matter of fact, difficult to simultaneously make the taper surface contact with the shaft end surface intersecting this taper surface. In this case, for ensuring an effective tool holding force, in a state where the taper hole 2 is brought into contact with the taper shank portion 4, further, a pre-load has to be given by making negative a magnitude of a gap .delta. between the end surface 3a of the tool holder 3 and the end surface 1a of the spindle shaft 1. Besides, the contact between the tapers has to be managed within a highly strict range where the magnitude thereof is on the order of 2 .mu.m. However, the tool holder 3 is replaced every time the type of the tool differs, and, hence, a multiplicity of tool holders 3 are required to be used. It is quite difficult to strictly manage the gap from the spindle shaft 1 with respect to all kinds of those toll holders 3. Consequently, there still exists a problem remaining unsolved, wherein an interchangeability of the tool holder 3 is not improved.